Passive-matrix type liquid crystal display apparatus and...

Computer graphics processing and selective visual display system – Plural physical display element control system – Display elements arranged in matrix

Reexamination Certificate

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C345S097000

Reexamination Certificate

active

06177919

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to a passive-matrix type liquid crystal display apparatus, and more particularly relates to a passive-matrix type liquid crystal display apparatus using ferroelectric liquid crystals and a drive circuit thereof.
BACKGROUND OF THE INVENTION
FIG. 20
shows the structure of a drive circuit of a passive-matrix type liquid crystal display apparatus disclosed in Japanese Publication for Unexamined Patent Application (Tokukaihei) No. 6-18848 (1994). In this structure, two analog switches are provided for each segmented electrode Yi (i=1 to m). For instance, when an analog switch
261
connected to a segmented electrode Y
1
is turned ON, a voltage V
3
is applied to the segmented electrode Yi. In other cases, an analog switch
241
is turned ON, and a voltage V
1
is applied to the segmented electrode Yi.
FIG. 21
shows a drive circuit on the source side of an active-matrix type liquid crystal display apparatus disclosed in Japanese Publication for Unexamined Patent Application (Tokukaihei) No. 5-100635 (1993). In this drive circuit structure, a voltage to be applied to the active-matrix type liquid crystal display apparatus is determined by selecting a power source from external power sources V
0
to V
7
according to the values of high order bits D
1
to D
3
of data retained in a latch circuit
201
, and by turning ON/OFF an analog switch
206
according to the values of low order bits D
4
and D
5
and a timing control signal
209
.
FIGS. 22 and 19
are block diagrams showing examples of the structure of a drive circuit of a conventional ferroelectric liquid crystal display apparatus and the waveforms of drive voltages. A drive circuit
71
on the scanning side, for driving scanning electrodes L, includes a shift register
76
and an analog switch array
77
. The drive circuit
71
selects one voltage waveform from three voltage waveforms, V
CA
, V
CB
and V
CC
, shown in
FIG. 19
, based on the value of input two-bit data YI, and applies the selected waveform to the scanning electrodes.
A drive circuit
72
on the segment side, for driving segmented electrodes S, includes a shift register
73
, a latch
74
, and an analog switch array
75
. The drive circuit
72
selects one voltage waveform from two voltage waveforms, V
SD
and V
SE
, shown in
FIG. 19
, based on data XI retained in the latch
74
, and applies the selected waveform to the segmented electrodes.
FIG. 23
shows an example of the waveforms used by a drive scheme for a ferroelectric liquid crystal display apparatus, which was suggested by the present inventors and disclosed in Japanese Publication for Unexamined Patent Application (Tokukaihei) No. 8-50278 (1996). In this drive scheme, a multiple gray scale display is achieved with a ferroelectric liquid crystal display apparatus by forming one pixel from three sub-pixels A
ijA
, A
ijB
and A
ijC
which are driven by three lines of scanning electrodes L
iA
, L
iB
and L
iC
(i=0, 1, . . . ), and one line of segmented electrode S
j
(j=0, 1, . . . ) as shown in
FIG. 11
, and by applying different selection voltage waveforms V
CA
, V
CB
and V
CC
shown in
FIG. 23
, to the three lines of scanning electrodes, respectively.
However, the above-mentioned conventional structure suffers from the following drawbacks.
In the drive circuit on the segment side of the conventional passive-matrix type liquid crystal display apparatus, two analog switches are required for one output level of each output terminal. When forming the drive circuit as an integrated circuit, if the number of output terminals and the area of a chip are fixed, the area for each analog switch can be increased by decreasing the number of analog switches per output terminal, and the output resistance of each output terminal can be reduced by an amount corresponding to the increase.
The technique disclosed in Japanese Publication for Unexamined Patent Application (Tokukaihei) No. 5-100635 (1993) above is a technique for decreasing the number of analog switches per output terminal in the drive circuit on the source side of the active-matrix type liquid crystal display apparatus. However, a scheme to obtain a plurality of output levels using one analog switch as a drive circuit of a passive-matrix type liquid crystal display apparatus has not yet been proposed.
A conventional ferroelectric liquid crystal display apparatus achieves bright and dark displays by using bistability of ferroelectric liquid crystals and aligning the molecule long axis of a ferroelectric liquid crystal molecule in one of the stable states with the polarization axis of a polarizing plate. With the use of ferroelectric liquid crystals with negative dielectric anisotropy, the memory angle changes depending on the root-mean-square value of a bias voltage applied, and the memory angle becomes larger with an increase in the root-mean-square value.
For example, in the drive scheme disclosed in Japanese Publication for Unexamined Patent Application (Tokukaihei) No. 8-50278 (1996) above, four kinds of voltage waveforms are applied to the segmented electrodes, and a variation of the root-mean-square value of the bias voltage is relatively wide. Therefore, even when the polarizing plate is positioned according to one bias state, if the display pattern changes into another bias state, the memory angle changes and the contrast is lowered.
The memory angle of the ferroelectric liquid crystal also changes depending on temperature. Therefore, even when the polarization axis of the polarizing plate is positioned according to a memory angle at a certain temperature, the memory angle varies as the temperature changes, and the contrast is lowered.
SUMMARY OF THE INVENTION
The first object of the present invention is to provide a segment drive circuit of a passive-matrix type liquid crystal display apparatus, capable of giving a plurality of output levels using one analog switch.
The second object of the present invention is to reduce the variation of the root-mean-square value of bias voltage and to prevent a lowering of the contrast in a ferroelectric liquid crystal display apparatus providing a multiple gray scale display using more than one kind of voltage waveforms.
The third object of the present invention is to compensate for a change in the memory angle of ferroelectric liquid crystals caused by a change in temperature, and to prevent a lowering of the contrast.
In order to achieve the first object, a passive-matrix type liquid crystal display apparatus of the present invention includes a pixel formed by a liquid crystal lying at each intersection of scanning electrodes and segmented electrodes, and is constructed so that a single analog switch is connected to each segmented electrode, a first electric potential is generated in the segmented electrode through the single analog switch connected thereto, by bringing the single analog switch into a conductive state and injecting a predetermined electric charge into the segmented electrode, and an electric potential different from the first electric potential is generated in the segmented electrode by bringing the single analog switch into a non-conductive state and adjusting a voltage to be applied to the scanning electrode.
In this structure, first, the analog switch is brought into a conductive state and a predetermined electric charge is injected into the segmented electrode so as to cause the segmented electrode to have the first electric potential corresponding to the predetermined electric charge. Next, when the analog switch is brought into a non-conductive state, since the electric charge does not enter into nor leave from the segmented electrode connected to this analog switch, the predetermined electric charge is retained on the segmented electrode.
The predetermined electric charge, Q, is given by
Q=CV
where C is the capacity of liquid crystal located between the scanning electrode and the segmented electrode, and V is the potential difference between the scanning electrode and the segmented electrode. Since Q

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